1. Field of the Invention
The present invention relates to an apparatus for porosimetric measurements, that is to measure the volume and size of microcavities present on the surface of a solid substance.
Said measurement is performed according to Drake method which essentially consists in placing a sample of the solid under test in a vessel, creating vacuum in said vessel, filling same with mercury and submitting mercury to ever increasing pressures. For each determined value of the pressure exerted on mercury, it is possible to establish, by means of a known formula, the pore average radius.
The biunivocal correlation between the pore volume and the relevant average radius is obtained by performing, for each value of pressure and therefore of the average radius, a detection of the volume variations of mercury present in the vessel. In fact, as this liquid is incompressible, each decrease in mercury volume which can be recorded outside corresponds to the introduction of an equal quantity of mercury into the sample pores.
2. Description of the Prior Art
In an actual application of said method, the vessel consists of a glass ampoule, or penetrometer, provided with an upwardly extending capillary and housed in an autoclave into which a liquid is introduced at ever increasing pressures, said liquid penetrating also partly into the ampoule mouth.
According to a known technique (U.S. Pat. No. 4,272,983) the detection of mercury volume variations in the penetrometer can be advantageously performed by recording the capacity variations of a cylindrical condenser, the internal armature of which is constituted by the same mercury contained in the capillary of said penetrometer, the dielectric is constituted by the capillary walls and the external armature is constituted by a metal lamina wound up around the external walls of the capillary. The capacity variations of such condenser, corresponding to the variations of its internal armature due to variations of the mercury meniscus level in the capillary, are proportional to the mercury volume penetrated, for a given pressure, into the pores of the solid sample under test.
According to this technique the variations of mercury volume and hence the determination of the pore volume are measured with very high precision and by means of a simple and functional device.
However, it has been noticed that the unavoidable size differences existing between a penetrometer and another one affect the value of their dielectric constant, which therefore results to be different for each penetrometer used. This difference between dielectric constants represents an important drawback considering that, in order to allow the performance of several subsequent analyses without being forced to empty and clean the penetrometer for each new analysis, different penetrometers are used, for each of them being therefore necessary to know the value of the dielectric constant. Moreover, each time a penetrometer is inserted into the autoclave it is necessary to perform heavy calibrations of the measuring apparatuses according to the value of the dielectric constant characteristic of said specific penetrometer.
The difference between the values of the dielectric constants is due to the difficulty of manufacturing penetrometers having capillaries with perfectly identical thickness, as well as to the difficulty of surperfectly rounding and in the same way the capillaries with the metal lamina forming the condenser armature. in particular the metal lamina does not always completely adhere to the external wall of the capillary and the different empty spaces created between the lamina and capillary of each penetrometer affect the value of its dielectric constant. Furthermore, such condenser, involving the need of obtaining penetrometers having perfectly calibrated thickness of capillary wall and internal diameter, is considerably expensive and increases the operating costs of the porosimeter.